Boolean modifier union doesn't work
You have to check the normals on your mesh (they should collectively point in one direction with respect to the surface) and it has to be a closed mesh (check for holes). Otherwise the modifier gets confused on what is on the outside and what is on the inside.

In this specific case the mirror modifier on the centerpiece of the steering wheel doesn't merge the vertices at the mirror axis. This results in the mesh being open in the center, thus confusing the boolean modifier. 

Solution: 

Increase merge limit on the mirror modifier to 0.1 or move the vertices in the center closer to the mirror axis